Machining of small bore metal tubing



June 10, 1969 P. M. F. WATSON 3,448,548

MACHINING Of SMALL BORE METAL TUBING Filed Oct. 15, 1965 INVENTOR; PETERM. F. WATSON United States Patent Int. Cl. B24]: 1/00 US. Cl. 51-281 4Claims ABSTRACT OF THE DISCLOSURE Method of forming the point of asurgical needle by grinding the end of a metal tube while the tube isfilled with a core of ice to avoid the formation of burrs.

This invention relates to the machining by grinding of capillary orother small bore metal tubing. In particular, the invention isapplicable to the formation of the points of tubular surgical needles,such as are used for the injection of medicaments or the withdrawal ofbody fluids, by obliquely grinding away the end of a steel tube ofappropriate dimensions.

If the walls of the tube are left unsupported during the grindingoperation, extensive burring of the metal takes place. The burrs thusformed have to be removed by subsequent operations, since if left inplace they would (in the case of a surgical needle) obstruct the bore orlumen of the needle, cause pain to the patient when the needle was used,and might break off within the body. In the case of tubes used for otherpurposes, burrs formed by grinding can obstruct the bore of the tube andif they subsequently break away they can interfere with proper operationof the apparatus in which the tube is used.

De-burring of the ground point of a tubular surgical needle is usuallyeffected by directing onto the point a stream of fluid loaded withabrasive particles, and/or by the use of steel brushes. Such abrasivede-burring operations inevitably cause some rounding off of the sharpcutting edges of the needle as formed by the grinding operation, thusblunting the needle.

Burr formation can be prevented or reduced by effecting the grindingoperation While the bore of the tube is filled with a core of metal,such a aluminium or brass This metal core serves to support the wall ofthe tube against the action of the grinding wheel, allowing the metal ofthe wall to be cut cleanly away without forming burrs. However, thisexpedient suffers from the disadvantages that the core metal which isground away tends to clog the grinding wheel and that the necessaryoperation of removing the portion of the core which remains within thebore of the tube after grinding is difficult to perform and may resultin damage to the ground edges.

In accordance with the present invention the formation of burrs isprevented or reduced to within acceptable limitsby effecting thegrinding operation while the bore of the tube is filled with a core ofice. Surprisingly, it has been found that a hard ice core is extremelyeffective for the purpose in question, while the ice chips or meltedwater produced from the core during grinding are without effect on thegrinding wheel and subsequent removal of the residual core from the tubepresents no difficulties.

Patented June 10, 1969 ice The figure of the drawing is an elevationalview partly in section.

In one way of practising the invention, in its application to themanufacture of tubular needles as illustrated in the drawing, a lengthof stainless steel tubing 10, of the dimensions required to form thedesired needle, is secured in a work-holding clip 11 with the end of thetube which is to be sharpened projecting from the clip. Water underpressure is forced into a clamped tube to fill its bore completely, thewater being retained within the tube by capillary attraction. The waterused may be distilled water, or it may contain additives such as thoseemployed in the coolant used in the subsequent grinding operation, butpreferablythe amount of any additions is limited so that the freezingpoint will not be depressed by more than a few degrees. The clipcontaining the water-filled tube is immersed in solid carbon dioxide ata temperature of C. for about one minute, or for any longer convenientperiod, to freeze the water into a hard core 12. Within the tubes. Verymuch lower temperatures, such as those obtainable with liquid air ornitrogen, may be used if desired to increase the resistance of the coreto melting, but it has been found that the temperature specified issuificient, provided that the grinding operation is carried out promptlyafter the withdrawal of the clip from the refrigerant and with properprecautions to prevent premature melting of the core.

Grinding is effected by engaging the exposed end of the clamped tubewith a grinding wheel 13, the relative disposition of the tube and thegrinding wheel being chosen to give the desired point configuration. Itis usually preferred to supply coolant liquid through a nozzle 14 to thezone where the tube contacts the grinding wheel. The coolant liquidemployed may be at room temperature, but the flow of coolant may need tobe limited to restrict conduction of heat to the tube and avoidpremature melting of the ice core. It is possible to employ a copiousflow of refrigerated coolant, for example at a temperature of 5 C. Thegrinding operation must, of course, be carried out sufficiently promptlyafter removal of the tube from the refrigerant, sufficiently rapidly andotherwise under such conditions that the ice core in the tube remainsunmelted until the grinding of the tube has been completed.

The pointed tube may then, if desired, be subjected to further grindingoperations to form a bevelled facet on one or both sides of the point.The bore of the tube may be filled wih an ice core during these bevelgrinding operations also, but this appears to be unnecessary, thetendency to burr formation being small.

The pointed needle obtained by the operations described is so free fromburrs that no abrasive de-burring operation is necessary to render itsuitable for use; any small adherent particles of metal resulting fromthe grinding operation which may be present are readily removed by theconventional cleaning operations in liquid baths to which the needle issubjected after grinding. The cutting edges of the needle accordinglyremain in their sharp, as-ground condition.

I claim:

1. A method of machining a small bore metal tube having an endcomprising the steps of filling the tube with water, freezing the waterwithin the tube into a hard core, and grinding the end of the tube to adesired configuration.

the tube.

2. A method according to claim 1, in which coolant liquid at atemperature below 0 C. is supplied to the end of the tube duringgrinding.

3. A method according to claim 1 wherein the tube is of capillary sizeand the Water is retained therein during the freezing step by capillaryattraction.

4. A method according to claim 1 including the step of forcing Waterunder pressure into the tube to fill the bore completely prior tofreezing the Water Within References Cited UNITED STATES PATENTS Heibelet a1. 51-283 Neidorf 51-283 Scholl 51-281 X Gersbach 51-277 X Arguesoet al 29423 Christen et a1. 51-283 Burns 29424 FOREIGN PATENTS GreatBritain.

US Cl. X .R.

